1. Field of the Invention
The present invention relates to a process for producing a carbon fiber by using pitch as starting material, and more particularly to a process for infusibilizing pitch fiber which comprises subjecting a pitch fiber to an oxidizing treatment to convert it to an infusible fiber.
2. Brief Description of the Prior Art
Recently, the process for producing a carbon fiber by using pitch as starting material has been watched with interest. The merit of this process consists in that a less expensive carbon fiber can be produced by it because pitch is less expensive than the starting materials of the prior processes such as PAN (polyacrylonitrile) and rayon, that a carbon fiber of high strength and high elasticity can be produced without carrying out the complicated stretching treatment in the firing process if a liquid crystal is used as the starting material of spinning, and that yield of carbonization is high. Thus, it is actively being studied and developed today.
The production of carbon fiber using pitch as starting material generally starts from the preparation of spinning pitch. Thus, crude coal tar pitch, petroleum pitch or the like, used as raw material of the process, is subjected to various treatments such as distillation, heat treatment, filtration, hydrogenation, fractionation using solvent, and the like either alone or in combination to remove the components obstructing the spinning process, such as low-boilding point volatile components, insoluble solid components and the like, from the pitch, to homogenize the quality of pitch and to make its quality appropriately heavy. Thus, an optically isotropic or optically anisotropic spinning pitch is obtained. The properties of a spinning pitch can be evaluated by measuring various parameters such as softening point, melt viscosity, optical structure, composition revealed by solvent fractionation, etc., and various spinning pitches different in properties can be used for spinning. Fundamentally, however, it is important that the spinning pitch contains no solid nor gas under the conditions of spinning and has a uniform flow property.
In the next stage, the resulting spinning pitch is formed into fiber to obtain a pitch fiber. Usually, melt spinning is suitable for producing a continuous long fiber, and centrifugal spinning is suitable for producing cotton-like short fiber or producing a drawn and arranged assembly of medium fiber having a medium length, i.e. sliver or two. The spinning temperature, hall number, drawing rate, stretch ratio, etc. may be selected appropriately so as to meet with the desired purpose. The spun pitch fiber usually has a fiber diameter of about 5 to 30.mu. (microns). If the fiber diameter is too great, the fiber properties are apt to deteriorate. If the fiber diameter is too small, an economical spinning process is difficult to secure.
In converting the pitch fiber into a carbon fiber, the thermoplastic pitch fiber must be subjected to an oxidative treatment prior to the carbonization with heat, by which the pitch fiber is converted to an infusible fiber or a fiber which does not fuse even if heated (the so-called infusibilizing treatment). Usually, the infusibilization is achieved by subjecting a pitch fiber to an addition reaction of oxygen or an oxidizing substance and thereby crosslinking the pitch molecules. For this purpose, various oxidizing gases and liquid or solution-formed oxidants have hitherto been proposed. Since this type of reactions progress from the surface of fiber, a pitch fiber of smaller diameter is expected to be infusibilized more rapidly. In the infusibilizing process, the pitch fiber is handled either in the form of being rolled and packaged, or in the form of being stretched continuously, or in the form of being accumulated on conveyer or in basket. An appropriate form may be selected depending on the intended final form of the fiber.
Next, the infusibilized fiber is heated in an inert gas at a temperature of about 600.degree. to 3,000.degree. C. to convert it to a carbon fiber (carbonization; when the temperature of this treatment is higher than 2,000.degree. C., it is sometimes called "graphitization"). By this treatment, the volatile components present in the infusibilized fiber and the part having a thermally instable structure in the pitch molecule are decomposed and vaporized off, and the aromatic ring structure in the molecule is grown. Thus, the fiber becomes rich in carbon content and sometimes becomes close to graphite crystal, and there is obtained a carbon fiber having high strength and elastic modulus.
For practising the heating, hot air oven, electric furnaces using various heating elements, plasma furnace and the like can be used. Since a large quantity of energy is consumed in any of these cases because of the high temperature, it is necessary to carry out the carbonization with a high efficiency. If desired, the carbonization may be carried out in two stages (low and high temperatures) or in more stages.
If desired, the carbon fiber thus obtained is further subjected to surface treatment, oiling, unwinding, and sometimes cutting, fibrillation, etc. However, these treatments will not be mentioned herein because of their generality.
All the above-mentioned processes are important for producing a carbon fiber. Among them, the infusibilizing step usually takes a long period of time and various troubles deteriorating the performances of carbon fiber often occur in this step. Accordingly, effective practice of this process is quite important to an economical production of carbon fiber.
The infusibilization is carried out for the purpose of oxidizing the thermoplastic pitch fiber to convert it into an infusible fiber having no thermoplasticity and thereby preventing the softening and deformation of the fiber in the subsequent carbonizing step. For achieving this purpose, a pitch fiber is usually heat-treated and oxidized while slowly elevating its temperature in an oxidizing gas. If control of this reaction is unsatisfactory, an uncontrollable reaction takes place to incur melting, inflammation, etc. Even if such uncontrollable reactions do not take place, a phenomenon called "sticking" often takes place to make the practice of this process difficult. As used herein, the term "sticking" means such a phenomenon that, in the infusibilizing process, adjacent pitch fibers are softened and deformed or sometimes a third material adheres to the contact area of the plural pitch fibers and, as its result, the pitch fibers are fixed together.
In a sticked pitch fiber, the fibers keep fixed after the subsequent carbonization, so that it lacks flexibility and its commercial value is damaged greatly. Sometimes, such a fiber has no commercial value at all.
The sticking phenomenon is apt to occur when pitch fiber is handled in the form of a tow or a strand. The handling of pitch fiber in the form of tow or strand is most suitable for the production of continuous filament, and it is quite difficult industrially to obtain a continuous carbon fiber of high quality by other methods such as drawing and arranging cotton-like or wool-like pitch fiber after infusibilization or after carbonization. Viewed from another angle, however, the infusibilization of pitch fiber in the form of tow or strand is not advantageous in the point of prevention of sticking. For, in the form of tow or strand, pitch fibers are bundled at a high density and have many contact points successively in the longitudinal direction. If a pitch fiber is heated in such a state for the purpose of infusibilization, the softened pitch fiber is readily stuck together at every contact point. In addition, the heat generated by the oxidation of pitch is accumulated in the tow or strand, which locally elevates the temperature of tow or strand and induces melting and sticking of mutually contacted pitch fibers. Further, the volatile substances evaporated from pitch fiber or the substances exuding out of pitch fiber cannot be rejected outside the fiber bundle but they are accumulated at the contact points, which acts as a sort of binder to cause the sticking.
Regarding the infusibilization of pitch fiber, a variety of techniques have hitherto been proposed. They include the method using a solution of oxidant (for example, Japanese Patent Publication No. 21,904/72, Japanese Patent Publication No. 21,905/72, etc.), the method using an oxidative gas (for example, Japanese Patent Publication No. 42,696/73, Japanese Patent Kokai (Laid-Open) No. 75,828/74, etc.), the combined use of the above-mentioned two agents (for example, Japanese Patent Kokai (Laid-Open) No. 88,729/76, Japanese Patent Kokai (Laid-Open) No. 30,915/84, etc.), and the like. However, the effect which these techniques exhibit is predominantly a shortening of the period of time required for infusibilization, and none of these methods is satisfactory from the viewpoint of preventing the sticking of pitch fibers. Further, the use of an oxidant such as hydrogen peroxide, chromic acid and the like is undesirable from the viewpoint of the safety of the process.
As a method for preventing the sticking of pitch fiber strands, combined use of a water-soluble oxidant, a water-soluble surfactant and finely powdered graphite has also been proposed (Japanese Patent Kokai (Laid-Open) No. 128,020/80). However, this technique also uses an oxidant, and therefore it is not desirable from the viewpoint of safety, as has been mentioned above.
Accordingly, an object of the present invention consists in providing a process for the infusibilization of pitch fibers having an effect of preventing the sticking of tow-formed or strand-formed pitch fibers at the time of infusibilizing treatment. Another object of the invention consists in providing a process for the infusibilization of pitch fibers exhibiting the above-mentioned effect without using those oxidants which are dangerous from the viewpoint of safety.
The process of the invention having the above-mentioned effect is surprisingly simple. Thus, the objects of the invention can be achieved by treating a pitch fiber with a dispersion of a finely powdered solid lubricant in water or a solvent prior to the infusibilizing treatment (at an appropriate time selected out of the period from the prevention of melting to the infusibilization) and heat-treating the pitch fiber to which the finely powder of solid lubricant adheres in an oxidative gas to perform the infusibilization.